Protein lysine methylation is a prevalent post-translational modification (PTM) and plays critical roles in all domains of life. However, its extent and function in photosynthetic organisms are still largely unknown. Cyanobacteria are a large group of prokaryotes that carry out oxygenic photosynthesis and are applied extensively in studies of photosynthetic mechanisms and environmental adaptation. Here we integrated propionylation of monomethylated proteins, enrichment of the modified peptides, and mass spectrometry (MS) analysis to identify monomethylated proteins in Synechocystis sp. PCC 6803 (Synechocystis). Overall, we identified 376 monomethylation sites in 270 proteins, with numerous monomethylated proteins participating in photosynthesis and carbon metabolism. We subsequently demonstrated that CpcM, a previously identified asparagine methyltransferase in Synechocystis, could catalyze lysine monomethylation of the potential aspartate aminotransferase Sll0480 both in vivo and in vitro and regulate the enzyme activity of Sll0480. The loss of CpcM led to decreases in the maximum quantum yield in primary photosystem II (PSII) and the efficiency of energy transfer during the photosynthetic reaction in Synechocystis. We report the first lysine monomethylome in a photosynthetic organism and present a critical database for functional analyses of monomethylation in cyanobacteria. The large number of monomethylated proteins and the identification of CpcM as the lysine methyltransferase in cyanobacteria suggest that reversible methylation may influence the metabolic process and photosynthesis in both cyanobacteria and plants.

蛋白质赖氨酸甲基化是一种普遍的翻译后修饰（PTM），在生活的所有领域中都起着至关重要的作用。然而，其在光合生物中的范围和功能仍然是未知的。蓝细菌是进行氧光合作用的一大类原核生物，广泛用于光合作用机制和环境适应性研究。在这里，我们整合了单甲基化蛋白的丙酰化，修饰肽的富集和质谱（MS）分析，以鉴定Synechocystis sp。中的单甲基化蛋白。PCC 6803（集囊藻）。总体而言，我们在270种蛋白质中鉴定出376个单甲基化位点，其中许多单甲基化蛋白质参与光合作用和碳代谢。我们随后证明了CpcM（一种先前在集胞藻中鉴定的天冬酰胺甲基转移酶）可以在体内和体外催化潜在的天冬氨酸氨基转移酶Sll0480的赖氨酸单甲基化，并调节Sll0480的酶活性。CpcM的损失导致初级光系统II（PSII）中最大量子产率的降低以及集胞藻光合作用过程中能量转移的效率降低。我们报告了光合生物中的第一个赖氨酸单甲基化，并提出了一个重要的数据库，用于对蓝细菌中的单甲基化进行功能分析。大量的单甲基化蛋白和CpcM作为蓝细菌中赖氨酸甲基转移酶的鉴定表明，可逆甲基化可能影响蓝细菌和植物的代谢过程和光合作用。